The present invention relates to a golf club and more particularly to a wood-type golf club including a head with a hollow outer shell construction.
Generally, there are known golf clubs in which a heavy material is disposed in the interior of a head thereof so as to improve the directionality of a ball hit thereby. For example, there is disclosed in Patent Document No. 1 a golf club in which a heavy material is disposed on a back side of a head in such a manner as to extend in a toe-to-heel direction. According to a head like this, the center of gravity is not concentrated to a central area of a face but extends in the toe-to-heel direction, whereby an effect can be obtained that the directionality is still stabilized even in case a ball is hit by the head at a position which is offset slightly towards the toe or heel.    Patent Document No. 1: JP-A-60-153885
As shown in FIG. 1, a swing that is performed by a normal golf player shares the same series of key elements; setup, that is the basic address position, takeaway or backswing, downswing that occurs after the golf club reaches the top position to shift its movement from upward to downward, follow-through that occurs after impact of a ball, and finish. Then, it is considered that the ball can easily be caught at a predetermined position (the sweet spot) on the head when hit by stabilizing the locus or swing plane of the head from the takeaway to the impact point of the ball. In other words, unless the swing plane in the downswing to the impact point of the ball is stabilized, it becomes difficult to catch the ball at the predetermined position.
As to the swing plane, as shown in FIG. 2, it is generally said that a good swing plane is a swing plan that resides within a range defined between an axis S of the shaft resulting when the golf club is set to match its lie angle α at address and an axis S′ which connects the root portion of the neck of the golf player with the ball when viewed from the side of the golf player. Namely, an ideal swing path is attained by swinging the golf club in such a manner that a resulting swing plane resides within the range defined by the axis S and the axis S′, whereby a stable impact at the sweet spot can easily be realized.
Incidentally, when an unnatural force is applied during a swing to the impact point of the ball, there is caused a problem that the swing plane becomes unstable. This unnatural force is such as to result mainly from a centrifugal force exerted on the head that is attached to the shaft, and it is considered from this fact that a weight balance in the head is one of important factors needed to stabilize the swing plane.
As exemplarily shown in FIG. 3A, in the head of the golf club disclosed in Patent Document No. 1, a heavy material M1 attached to the head extends horizontally (when used here, “horizontally” means a direction which follows a horizontal plane P which constitutes a reference point when the head is set on the ground at its lie angle with the golf club set in its address position) in the toe-to-heel direction, and when a golf club like this is actually swung, the golf player feels during the downswing until the impact point is reached that a distal end side of the head is pulled. Namely, the realization of a stable swing plane is disrupted by the fact that the distal end side of the head is so pulled.
To explain this specifically, when considering a centrifugal force (a centrifugal force generated by the aforesaid weight distribution) exerted on the head portion in the midst of downsing, there is a tendency that the centrifugal force increases as the toe side is approached as shown in FIG. 3A. To explain this in a simplified fashion, in the aforesaid weight distribution, let a centrifugal force exerted on a position P1 on the toe side be F1, a centrifugal force exerted on a central position P2 be F2 and a centrifugal force exerted on a position P3 on the heel side be F3, such a relationship as F1>F2>F3 results. Assuming that the head draws approximately a centrifugal path, and let the angular velocity thereof be ω and a weight at each position be m, the centrifugal force exerted on the head is defined like F=mrω2 (r denotes a distance from a swing axis which is a center axis of the centrifugal path along a longitudinal direction of the shaft to a position where the weight exists).
In the aforesaid weight distribution, when considering that weight is evenly distributed in the toe-to-heel direction, while weights exerted on the respective points and the angular velocity are the same, in the heavy material that is distributed horizontally, distances r1, r2, r3 from the swing axis along the longitudinal direction of the shaft to positions where respective weight components exist become different from each other (r1>r2>r3). Namely, from the aforesaid equation, forces generated in the respective portions by the heavy material M1 become, as discussed above, F1>F2>F3 (the centrifugal forces exerted on the respective positions are shown by vectors in FIG. 3A, and the centrifugal force exerted on each position along substantially the longitudinal direction of the shaft becomes larger as the toe side is approached).
Then, the centrifugal force which differs in magnitude at the respective positions on the head (the force becomes larger as it approaches the toe side) acts such that an actual swing plane is caused to deviate from the ideal swing plane during downswing by the difference in centrifugal force so generated. Namely, since a component in a vertical direction X of a centrifugal force exerted along the longitudinal direction of the shaft due to the difference attempts to pull the head in a direction indicated by an arrow D1 in FIG. 3A, a force in a direction indicated by an arrow D1′ is exerted on a grip portion of the shaft and hence constitutes a factor which disrupts the stable swing plane. In addition, since a component in a horizontal direction Y of the centrifugal force attempts to pull the head in a direction indicated by an arrow D1 in FIG. 4, this component also constitutes a factor that disrupts the stable swing plane.
Then, this component in the horizontal direction Y shows a strong tendency that the head is likely to travel on an in-to-out path S1 relative to a target line T at the time of impact and hence comes to disrupt the implementation of a stable impact (ideally, while it is good to impact the ball on an in-to-in path S relative to the target line T, in the event that a force exerted on the heel side becomes too large on the contrary, there is increasing a tendency that the head travels on an out-to-in path S2, and hence this comes to disrupt the implementation of a stable impact).
Thus, with the weight-concentrated material provided in the head in such a manner as to extend along the reference horizontal plane P in the toe-to-heel direction, the golf player tends to feel a change in centrifugal force due to the horizontally extending weight distribution. Even in case this occurs, experienced or high-level golf players have a technique to stabilize the disturbed swing plane by consciously or unconsciously moving the writs or the like. However, golf players who have no such technique tend to strike the ball along an unstable swing plane that is caused by the change in the centrifugal force or strike the ball along a swing plane that has not yet be rectified or stabilized sufficiently.
In addition, even with general heads of golf clubs in which the heavy material described above is not provided, when compared with other heads having outer shell constructions, since the sole portion of the head is formed thick along the horizontal direction, its weight distribution resembles that of the heavy material M1 shown in FIGS. 3A and 3B in which weight is distributed to extend horizontally in the toe-to-heel direction, this causing a factor which disrupts the stable swing plane at the time of downswing.
It is considered from the description that has been made heretofore that the stable swing plane can be maintained in the swing to the impact point by constructing such that the force indicated by the arrow D1 in FIG. 3A and the force indicated by the arrow D2 in FIG. 4 are generated as little as possible in the head.